Modern wire-free data transmission systems are subject to ever more stringent requirements for the sensitivity of receiving amplifiers. On the one hand, this is because the transmission signal field strength is becoming ever lower in order to prevent interference in the adjacent channels, and thus interference with other mobile data transmission systems. On the other hand, the amount of information transmitted is becoming ever greater. This leads on the one hand to spectral broadening of the transmitted signal, that is to say a wider bandwidth is required for the transmitted signal. One typical example of a broadband mobile radio standard is the 802.11a or 802.11b (W-LAN) Standard, whose signals have a bandwidth of about 20 MHz at a carrier frequency of 2.4 or 5.2 GHz, respectively. Data transmission methods for digital television (digital video broadcast, terrestrial, DVB-T, digital video broadcast, satellite, DVB-S) use even wider bandwidths of several hundred Megahertz.
On the other hand, the modulation methods are becoming considerably more complex, and of higher quality. While, by way of example, the FSK (frequency shift keying) modulation method is used for the GSM mobile radio standard, and is relatively insensitive to amplitude and phase fluctuations, the 802.11a, 11b mobile radio standards and the DVB-T television standard use a so-called OFDM (orthogonal frequency-division multiplexing) modulation method. These Standards have a highly sensitive reaction to noise components in the received signal.
The noise in the received signal originates on the one hand from noise components in the frequency band of the received signal. This component is caused, for example, by atmospheric interference or by interference from other transmitted signals. On the other hand, the noise also contains a component which is caused by the receiving components that are used. For example, in principle, every active and passive component in the receiving chain of a receiver has a certain amount of self-noise. The components in the receiving chain, and in this case in particular the receiving amplifiers, add a noise component of their self-noise to the received signal, and thus change the ratio of the useful signal to the undesirable noise signal. One characteristic variable for components which indicates the component of the noise produced by the components is the noise factor. This is defined as the quotient of the signal-to-noise ratio of a signal at the input of the component, and the signal-to-noise ratio at the output of the element.
For receivers, it is necessary particularly for the first amplifier contained in the receiving chain to have a particularly low noise factor, and thus to add only a small amount of noise to the received signal. Furthermore, the receiving amplifier should have sufficiently good sensitivity over a wide bandwidth.
In order to solve this problem, some receiving amplifiers have an external “tracking filter” connected upstream of them, which essentially represents a tracking bandpass filter. This suppresses the undesirable spectral components outside the useful signal bandwidth. Some of the noise is also suppressed in this case. However, a tracking filter such as this has the disadvantage that the element which governs the frequency, for example a varactor diode, must be supplied with an adjustment voltage which is derived from a voltage controlled oscillator that is used in the receiving unit. This leads to high costs and to a large amount of space being consumed, as well as a complex subsequent calibration routine.